1. Field of the Invention
The present invention relates to an integrated multifunctional sensor for detecting arcing faults. More specifically, it relates an integrated multifunctional sensor for detecting arcing faults, which combines multiple detection phenomenologies to achieve high probability of detection and low false alarm rate.
2. Description of the Related Art
Arcing faults are essentially high-impedance short circuits in power supply systems. In more precise language, an arcing fault may be defined as a variable impedance sustained luminous discharge of electrical power across a gap in a circuit. These discharges conduct sufficient current to sustain an arc but remain below the trip threshold of circuit breakers. They typically start as inline high-resistance caused by a dirty or loose connection; this situation may be sustained for days or weeks. The heat from the faulty connection eventually melts the connection causing an in-line arc. The in-line arc then jumps phase to generate white-hot heat that melts and consumes the metal in switchgear in a few seconds.
An arc generates a searingly bright, white-hot light and a pressure shockwave. An arc also generates high-frequency harmonics on the power lines. Detectable arcs dissipate a significant amount of power. The current of an arc depends on the voltage available and the spacing of the conductors. While arcs can occur at household voltages and currents, these arcs do most of their damage due to the ignition of adjacent combustible material and are not the focus of this disclosure. Arcs in main power distribution centers occur at voltages of 400 V and above and disable the distribution centers due to bulk vaporization of metals. The power distribution center arcs are the main focus of this disclosure.
The rise time of an arc is in the nanosecond range. It generates light and high-frequency harmonics immediately. A shockwave travels at the speed of sound or about 340 m/s, and takes about 2.94 milliseconds to travel one meter from the arc. An arc sustained for a few hundred milliseconds begins to combust and to destroy copper and steel in power distribution switchboards. Testing has shown that if the arc is quenched within 250 milliseconds then the damage will not generally require the replacement of components of the switchboard. If the arc time extends to one second then collateral damage can include holes in the sheet metal wall of the switchboard. This defines a range of time between about 1 and 200 milliseconds within which a protective system must detect, discriminate and extinguish an arc before significant damage occurs.
Dirty and loose connections often are the genesis for arcing faults. The conditions for an arcing fault often take some time to develop. As dirt accumulates and connections loosen the circuit increases in resistivity; this, in turn, generates heat. The heat will bake off particulates of insulation from the conductors.
There are presently a variety of techniques and systems for detecting arcing faults. Below are listed the main techniques along with their deficiencies.
Arc-proof switchboards contain the damage but do not prevent it. They are constructed from heavier steel to reduce the likelihood of flying debris and they contain pressure relief panels in the top of the switchboards to vent the hot arc gases away from direct impingement upon personnel. Their high purchase price, high installation costs and the down time needed to install them, make arc-proof switchboards too expensive for use in existing installations.
Multi-function monitoring (MFM) works by attaching current transformers on every major cable entering or leaving a switchboard or a network of switchboards. A smart box sums all of the currents entering or leaving a circuit node. Any missing current is evidence of an arc and results in opening the protective breakers. Alternatively MFM systems sometimes look at noise on the power line or at the absolute value of the currents compared to some reference value. While these systems can be effective with cable arcs, they are much less effective on bus bars due to the wide variation in impedance bus bar geometries. Additionally in dense switchboard groups the wide range in size of loads makes it difficult to discriminate the currents lost to loads vs. that lost to an arc.
Current relay techniques have a long history in the electrical industry. Current transformers are attached to major conductors and then connected to the appropriate relays. If the currents in the various conductors of the circuit are out of a predefined balance the circuit is interrupted by the relays. This scheme can be useful to insuring a balance in the current between multiple loads but they have not proven to be effective against arcing. Additionally, consider that the current transformers and relays required for current relaying and for the MFM require that bulky expensive components be added to already cramped switchboards.
Arc fault circuit interrupters (AFCI) are useful only on low voltage circuits with amperage les than 20A. AFCIs work by looking at the frequency, duration, or pulsing of high frequency noise on a circuit due to low power sputtering arcs. While AFCIs work in household environments, they incur problems with discrimination between the noise from bad arcs and that of normal arcs due to switch openings, filaments blowing, hair dryers, etc. Manufacturers of AFCI generally believe that due to the discrimination issues AFCIs will never work at higher voltages or in an industrial environment.
ABB arc guard system has optical fiber technology coupled with or without current detection. Coupling the optical signal with a current threshold can cause the system to miss smaller arcs. The use of fiber optics restricts the angle of view of the sensors and worsens the sensitivity for smaller arcs. This system has no Built-In Test (BIT) capability; therefore one can not be sure that the system is on line and functioning correctly. It is geared to protecting individual switchboards and may not be set up to look at large switchboard networks in zone-oriented schemes. The arc guard system may also have no predictive capability.
Thermal imaging of electrical switchboards can identify faulty connections and components and direct preventive maintenance; unfortunately typically less than half of all connections are in view of the thermal imaging operator. Thermal imaging is only effective if performed while the switchboards are energized and up to their normal operating temperature. Thus the process requires working on energized switchboards which is difficult to perform and presents a safety hazard. Due to costs, thermal imaging is only done every one-two years; however it can only look forward a few days.
The Continuous Thermal Monitoring System (CTM) can prevent arcing faults due to overheated connections by the detection of pyrolysis products from the overheated connections. A CTM indications directs the operator to perform preventative maintenance in a given switchboard before an arc occurs. This system is not effective against arcs caused by contamination or falling objects.
A related patent is U.S. Pat. No. 4,658,322 Arcing Fault Detector, by Neftali Rivera. The arcing fault detection system disclosed in this patent comprises a plurality of temperature sensors and a differential pressure sensor, with their intelligence being processed by a fault protector which controls the tripping of the circuit breaker(s). This patent further discloses the optional use of photodiode(s), which may be used with or in place of the temperature sensors to detect light generated by an arc fault. While the system disclosed in this patent is capable of detecting arcing faults which are accompanied by pressure, temperature and/or light, the system has a high false alarm rate and other deficiencies.
In summary, while these sensors accomplish their intended purposes, their numerous serious deficiencies have been noted above, and there remains a strong need for an arcing fault detection system which has both a high probability of detection and a low false alarm rate for a broad range of amperages and fault-types, thus addressing and solving problems associated with conventional systems.